This invention relates generally to a method of operating a sand screw. The method of the present invention compensates for calibration errors which typically arise when only a selected calibration factor is used in controlling the operation of a sand screw.
A sand screw is a machine for conveying sand from an inlet where sand free falls out of a hopper onto an auger-like implement (i.e., the "screw") that moves the sand to an outlet of the sand screw. At least in the oil and gas industry, the outlet is typically above a tub so that the sand drops into the tub for mixing with a liquid pumped into the tub from another source. This is, broadly, a typical way in which a fracturing fluid is made prior to being pumped into a well for fracturing a formation to increase production of oil or gas out of the fractured formation. Once pumped downhole in the fracturing fluid, the sand becomes a propping agent which holds the formation open after it has been fractured by the hydraulic pressure of the fluid.
The mixture of the fracturing fluid is typically controlled using a computer for, in effect, calculating sand concentration of the mixture, comparing the calculated concentration with a desired concentration and generating a control signal to adjust the speed of the sand screw depending on any difference between the calculated concentration and the desired concentration. This control is based on the sensed flow of liquid into the tub and on the quantity or rate of sand moved by the sand screw as determined by the sensed speed of the sand screw and a selected calibration factor.
A transducer provides a signal indicating the sand screw speed, and the computer uses the calibration factor to convert the speed into a sand quantity or delivery rate. For example, if a transducer provides a series of pulses, the calibration factor typically specifies how many pulses per pound of sand; therefore, dividing the counted pulses by the calibration factor gives the number of pounds of sand delivered by the sand screw. Taking this over a selected time period gives the delivery rate. Alternatively, if the transducer signal were used to represent revolutions per minute of the screw, the calibration factor would specify how many pounds of sand are moved per revolution; therefore, multiplying the revolutions per minute by the calibration factor gives the delivery rate. Multiplying also by time would give the number of pounds of sand.
In the existing system I am aware of for operating a sand screw to control its speed so that a desired concentration can be obtained, one calibration factor is used for a given job. That is, a single number is used as the divisor or multiplier to obtain a quantity of sand or delivery rate of sand value regardless of the speed at which the sand screw is operating. This produces errors because a sand screw does not deliver sand at a constant rate throughout the range of speeds at which the sand screw can operate. This will be more fully explained below, but at present it is sufficient to note that such errors exist, and such errors can be detrimental because they can cause a mixture having an undesired actual concentration to be produced. In the example of the mixture being a fracturing fluid, such incorrect concentration can adversely affect the fracturing job and the resultant productivity of the well. Thus, there is the need for an improved method of operating a sand screw which compensates for such errors.